Non-Contact Forces

A non-contact force is defined as:

A force which acts at a distance, without any physical contact between bodies, due to the action of a field

Examples of non-contact forces include:
- Gravitational force
- Electrostatic force
- Magnetic force

Gravitational force, F_g

The attractive force experienced by two objects with mass in a gravitational field e.g the force between a planet and a comet
- Weight, on Earth, is the gravitational force of the Earth acting on an object with mass
  - $F_{g} = m g$
Electrostatic force, F_e
- A force experienced by charged objects in an electric field which can be attractive or repulsive e.g. the attraction between a proton and an electron
Magnetic force, F_m
- A force experienced between magnetic poles in a magnetic field that can be attractive or repulsive e.g. the attraction between the north and south poles of magnets

forces-at-distance-igcse-and-gcse-physics-revision-notes

Examples of non-contact forces

Worked example

A child drags a sledge behind them as they climb up a hill.

Describe the contact and non-contact forces acting on the child and the sledge.

Answer:

Step 1: Identify the contact forces acting on the child and the sledge

The child pulls on one end of the rope and the sledge pulls on the other end of the rope
- This force is tension
The ground pushes against the child and the sledge
- This is the normal contact force
The surface of the sledge moves over the the surface of the ground opposing the motion of the sledge
- This force is surface friction
The surfaces of the child's shoes move over the surface of the ground (enabling the child to walk)
- This force is also surface friction
The child and the sledge move through the air
- This force is fluid resistance or drag

Step 2: Identify the non-contact forces acting on the child and the sledge

The gravitational pull of the Earth acts on the child and the sledge
- This force is weight

Examiner Tip

You will often see weight as W rather than F_g, even on the IB exam papers. It is always best to stick with whichever symbols you have been given in the question. However, if no symbols are given in the question, use the correct symbols from the syllabus (F_g).

Non-Contact Forces (SL IB Physics)

Revision Note